This invention relates to a battery charging and supervision system for monitoring the state of a battery being charged and a series-connected protective fuse in order to detect malfunctions in operation. More specifically, the system of the invention monitors the presence and voltage of the battery, and the presence and continuity of the protective fuse.
There are many environments where a d-c operating voltage, required to power a load in an emergency, is derived from a rechargeable battery which is continuously being charged by a battery charger, the charger in turn being energized by the primary power source, such as an a-c line voltage source. An arrangement of this type provides a relatively stable, non-interruptible d-c voltage and is particularly well suited for powering computer equipment which usually requires the continuous application of an operating potential to operate properly. If there is a power loss, even though momentary, the operation of the computer equipment may be deleteriously affected.
In many cases it is desirable to supervise or monitor the rechargeable battery to make certain that the stand-by or backup d-c power supply for emergency use is functioning properly, and to provide a signal of some type when there is a malfunction. Such battery supervision is most helpful when the battery, and the load to be powered thereby, are incorporated in a security protection system or a fire detection system. A fuse is ordinarily connected in series with the battery and opens in the event that either the charging current or the load current exceeds a prescribed limit. It is thus desirable that the supervision system also monitor the condition of the protective fuse to determine that the fuse is actually connected in the circuit and is effective, namely, is continuous.
Monitoring a battery and a series-connected fuse is especially difficult when a d-c charging voltage is constantly applied to the series combination, since the charging voltage itself will interfere with testing of the battery and fuse. The battery voltage could be abnormally low or the fuse could be blown or open, and the monitor would provide a false test result indicating that the battery voltage exceeds the desired threshold level and that the fuse is not blown and is continuous. This problem has been overcome in a prior system by utilizing unfiltered, full-wave rectified a-c voltage for charging the battery, which is treated as a large storage capacitor. In the battery's presence and when its voltage is at the desired level, and when the fuse is present and is continuous, the output of the battery charger will essentially be a d-c voltage with very little ripple. In the battery's absence or when its voltage is abnormally low, or when the fuse is open, the ripple on the output of the battery charger will be very large. Thus, the state of the battery and fuse can easily be determined by a sensing circuit.
The problem remains, however, if the d-c charging voltage has no significant ripple component and has a relatively constant instantaneous amplitude, as is the case when the d-c voltage is derived from rectified a-c line voltage which has been filtered. The present invention is directed to the solution of the problem when a substantially ripple-free d-c charging voltage is employed. Successful monitoring of the battery and fuse is achieved, with such a d-c charging voltage, in the present invention by means of a unique supervision system of relatively simple and inexpensive construction.